Corrosion behavior of WC–Co hardmetals in the oil-in-water emulsions containing sulfate reducing Citrobacter sp.

•A special SRB strain of Citrobacter sp. was isolated from O/W emulsion.•The SRB degraded the emulsion and made it more aggressive to cobalt phase.•The biofilms seriously affected the corrosion behavior of WC–30Co in various media.•Either accelerated or inhibited corrosion occurred owing to microbial activities.

Many microbial issues have been widely proposed in the omnipresent metalworking fluids (MWFs). In this work, a particular sulfate reducing Citrobacter sp. strain was isolated from engaged oil-in-water emulsion, named as CK2. The occurrence of microbially influenced corrosion of WC–30Co hardmetals caused by CK2 in O/W emulsion and nutrient media was proved by electrochemical and micro-morphological analyses. Preferential cobalt loss determined the corrosion failure in biodegraded emulsions. CK2 made the emulsion more corrosive to hardmetals. Microbially influenced corrosion inhibition was observed in CK2-containing emulsion. In nutrient media, the corrosion rates of WC–30Co multiplied about 10 times due to microbial activities.